1. Field of the Invention
The present invention relates generally to a multi-hop relay Broadband Wireless Access (BWA) communication system, and in particular, to an apparatus and method for synchronously providing a direct link service and a relay link service in a multi-hop relay BWA communication system.
2. Description of the Related Art
One of the most critical requirements for deployment of a 4th Generation (4G) mobile communication system is to build a self-configurable wireless network. The self-configurable wireless network refers to a wireless network configured in an autonomous or distributed manner without control of a central system to provide mobile communication services. For the 4G mobile communication system, cells of very small radii are defined for the purpose of enabling high-speed communications and accommodating a larger number of calls. The conventional centralized wireless network design is not self-configurable. Rather, the wireless network should be built to be under distributed control and to actively cope with an environmental change like addition of new Base Stations (BSs). That is why the 4G mobile communication system requires the self-configurable wireless network.
For real deployment of the self-configurable wireless network, techniques used for an ad hoc network should be introduced to a wireless access communication system. Such a major example is a multi-hop relay BWA communication system configured by applying a multi-hop relay scheme used for the ad hoc network to a BWA network with fixed BSs.
In general, since a BS and a Mobile Station (MS) communicate with each other via a direct link, a highly reliable radio link can be established easily between the BS and the MS in the BWA communication system. However, due to the fixedness of the BSs, the configuration of a wireless network is not flexible, making it difficult to provide an efficient service in a radio environment experiencing a fluctuating traffic distribution and a great change in the number of required calls.
The above drawback can be overcome by a relay service that delivers data over multiple hops using a plurality of neighbor MSs or neighbor Relay Stations (RSs). The use of the multi-hop relay scheme facilitates fast network reconfiguration adaptive to an environmental change and renders the overall wireless network operation efficient. Also, a radio channel in a better channel status can be provided to an MS by installing an RS between the BS and the MS and thus establishing a multi-hop relay path via the RS. High-speed data channels can be provided to MSs in a shadowing area or an area where communications with the BS are unavailable, and cell coverage can be expanded.
FIG. 1 illustrates the configuration of a typical multi-hop relay BWA communication system.
Referring to FIG. 1, in the multi-hop relay BWA communication system, MSs 140, 150, 160 and 170 (MS1 to MS4) can receive BWA services through a BS 100, a primary RS (RS1) 110, and secondary RSs (RS2) 120 and 130.
MS1 and MS2 within the coverage area 101 of the BS 100 communicate with the BS 100 via direct links L1. MS2, which is located at the cell boundary of the BS 100 and thus placed in a poor channel state, can receive a higher-speed data channel via an RS-MS link L2 provided by RS2 130 compared to the speed via the direct link L1.
MS3 and MS4 outside the coverage area 101 of the BS 100 communicate with the BS 100 via RS-MS links L3 provided by RS1 110. The communication links between the BS 100 and MS3 and MS4 via RS1 110 expand the cell coverage. MS4, which is located at the cell boundary of RS1 110 and thus placed in a poor channel state, can increase its transmission capacity using an RS-MS link L4 provided by RS2 120.
As described above, when an MS is in a poor channel state at a cell boundary of a BS or in a shadow area suffering from a severe shielding effect due to, for example, buildings, the BWA communication system enables the MS to communicate with the BS by providing a better-quality radio channel to the MS via an RS. In other words, the BS can provide a high-speed data channel to the cell boundary and the shadow area and expand its coverage area by the multi-hop relay scheme.
The RSs 110, 120 and 130 are classified into RS1 (RS 110) that expands cell coverage and RS2 (the RSs 120 and 130) that increases capacity according to their operation capabilities.
As stated above, RS1 110 serves to expand the cell coverage of the BS 100. Because MS3 and MS4 have difficulty in receiving services directly from the BS 100, they acquire synchronization to the BS 100 and perform network entry to the BS 100 via RS1 110. Therefore, RS 110 provides functionalities for the initial access of the MSs 160 and 170, that is, provides a control channel (or traffic channel) and a random access channel to MS3 and MS4.
RS2 120 and RS2 130 relay services to MS2 and MS4 within their cell coverage area, for the purpose of increasing service capacity. MS2 receives a control channel and a random access channel from the BS 100 and a traffic channel from RS2 130. MS4 receives a control channel and a random access channel from RS1 110 and a traffic channel from RS2 120.
Typically, transmission/reception is carried out between a BS and an MS in frames having the configuration illustrated in FIG. 2 in the BWA communication system. FIG. 2 illustrates a Time Division Duplex (TDD) frame structure compliant with the Institute of Electrical and Electronics Engineers (IEEE) 802.16 standard, for data transmission/reception, between the BS and the MS.
Referring to FIG. 2, a TDD frame 200 is divided into a DownLink (DL) subframe 210 and an UpLink (UL) subframe 220 with a guard region called Transmit/receive Transition Gap (TTG) in between. A guard region called Receive/transmit Transition Gap (RTG) is interposed between TDD frames.
A preamble and a common control channel are included in mandatory slots of the DL subframe 210 and broadcast to the cell coverage area of a BS. MSs within the coverage area of the BS acquire synchronization and control information from the preamble and the common control channel.
As described above, RS1 provides BWA services to MSs or RSs which have difficulty in establishing direct links with the BS as they are located outside the coverage area. Therefore, RS1 must provide control information and initial ranging slots to the MSs or the RSs, for network entry as well as user traffic service. Especially, RS1 must provide a service via an indirect link in the same configuration as via a direct link as illustrated in FIG. 3, in order to ensure backward compatibility for the MSs.
For a relay service, after receiving a control channel or a traffic channel from a BS or an MS, an RS relays them. That is why the RS carries out both transmission and reception within a single-directional subframe.
FIG. 3 illustrates a TDD frame structure in a conventional multi-hop relay BWA communication system.
Referring to FIG. 3, a single-directional subframe 300 is divided into a direct link zone 301 and an indirect link zone 303, and another single-directional subframe 310 is divided into a direct link zone 311 and an indirect link zone 313. For a relay service, predetermined parts of the subframes 300 and 310 are allocated to the indirect link zones 303 and 313. Thus, an RS receives information and data for relaying in the direct link zones 301 and 311 and relays them in the indirect link zones 303 and 313.
For instance, RS1 receives control information and a traffic burst to be relayed from the BS or an MS in the direct link zone 301 or 311 and then relays them to the BS or the MS in the indirect link zone 303 or 313.
RS2 receives unicast traffic bursts to be relayed from an MS or the BS in the direct link zone 311 or 301 and then relays them to the MS or the BS in the indirect link zone 303 or 313.
When communications are conducted in the frame structure illustrated in FIG. 3, a different node that provides a BWA service has a different frame timing, i.e. different nodes operate asynchronously in the BWA communication system, as illustrated in FIG. 4.
FIG. 4 is a diagram illustrating transmission and reception timings of MSs in the conventional multi-hop relay BWA communication system.
Referring to FIG. 4, an MS that receives a direct link service from a BS has the timing of a BS frame 421 because communications are made in direct link zones 401 and 411. On the other hand, an MS that receives a relay link service from an RS has the timing of an RS frame 423 because communications are made in indirect link zones 403 and 413.
Typically, synchronization and handover are carried out based on control information and preamble transmitted to a fixed position in a frame in the BWA communication system. However, since MSs operate asynchronously depending on the subject that provides them with service as illustrated in FIG. 4, the handover and synchronization are very difficult as illustrated in FIG. 5.
FIG. 5 illustrates signal flows when MSs move in the conventional multi-hop relay BWA communication system.
Referring to FIG. 5, when an MS 520 (MS1) moves from a BS 500 to the coverage area of RS1 510 while receiving a service from the BS 500, MS1 must receive a preamble and control information from RS1 510.
When an MS 530 (MS2) moves from RS1 510 to the BS 500 during receiving a service from RS1 510, MS2 must receive a preamble and control information from the BS 500.
As illustrated in FIG. 3, however, the BS 500 and RS1 510 operate asynchronously, thereby making it difficult for MS1 and MS2 to acquire the preamble and the control information after the handover.
Moreover, since the BS provides a service to the MSs and the RS simultaneously in a direct link subframe, as illustrated in FIG. 3, the freedom of configuring a system in which the BS supports the RS is low.